Engine emissions control systems may use a lean NOx trap to reduce nitrogen oxide (“NOx”) emissions. Generally, a lean NOx trap is a device that stores NOx compounds present in lean exhaust (i.e. exhaust that has an excess of oxidants relative to reductants). Periodically, rich exhaust may be provided to the lean NOx trap to reduce NOx compounds stored in the lean NOx trap, thereby regenerating the trap. Lean NOx traps may also variously be referred to as NOx storage-reduction catalysts, or simply NOx catalysts.
Engine emissions control systems also may use a diesel particulate filter (DPF) or a catalyzed diesel particulate filter (CDPF) to reduce particulate emissions. Such filters may be periodically regenerated when the particulate matter builds up to levels where pressure drop across the filter exceeds a specified level. Active regeneration of a loaded DPF or CDPF can take several tens of minutes and may require that lean exhaust having a considerable excess of oxygen (over reductants) be supplied to the DPF or CDPF for the duration of the regeneration process to ensure sufficient oxygen to sustain combustion of the particulate matter. However, catalyst deactivation of the lean NOx trap may occur during the higher temperature lean operation of DPF or CDPF regeneration. Where engine shutoff occurs during DPF/CDPF regeneration or during other periods of lean operation, a lean NOx trap may be left in the deactivated state.
In one approach, the above issues may be at least partially addressed in an apparatus having an engine, an exhaust conduit and a lean NOx trap disposed along the exhaust conduit by detecting an engine shutoff, determining an engine stop position, and creating a reductive environment in the exhaust conduit upstream of the NOx trap at a time based upon the engine stop position. This may create a reductive environment in the NOx trap during engine shutoff, and therefore may help to at least partially reactivate the lean NOx trap during engine shutoff.